The present invention relates to a solution for removing an aluminum oxide film and a surface treatment method for aluminum or an aluminum alloy, particularly to a solution for removing an aluminum oxide film and a surface treatment method for aluminum or an aluminum alloy which are effective when used in or as a pre-treatment in forming a UBM (under-bump metal) or bumps on a wafer by plating.
As a method for forming a UBM or bumps on a silicon wafer, there has been used a method including the steps of subjecting aluminum thin film electrodes patterned on a wafer to a zinc substitution treatment so as to form a zinc film and then performing electroless plating to form bumps, a method including the steps of conducting a palladium treatment in place of the just-mentioned zinc substitution treatment and then performing electroless plating to form bumps, or a method including the steps of directly replacing the surfaces of aluminum thin film electrodes with nickel and then performing autocatalytic electroless plating to form bumps.
Here, irrespectively of the method selectively used to form the UBM or bumps, a degreasing treatment of the aluminum thin film electrodes, a treatment for removing an aluminum oxide film, metallic impurities or the like present on the aluminum thin film electrodes or the like treatment is ordinarily conducted as a pretreatment. In this case, an aluminum oxide film produced in an extremely small thickness upon immersion in nitric acid or the like can be subjected directly to a plating treatment in the subsequent step to thereby achieve the desired plating without any problem. On the other hand, in the case where a rigid aluminum oxide film generated through such a manufacturing step as a grinding step and an annealing step is left on the surface of the workpiece, the plating film formed in the subsequent step may show insufficient adhesion or pits may be formed in the plating film, and, in worse conditions, the desired plating film may not be deposited successfully. Therefore, it is desirable to remove such a rigid aluminum oxide film completely before plating.
To cope with such a problem, a method in which a substrate for plating is formed by a dry process without conducting dissolution of the aluminum oxide film has been proposed (refer to Japanese Patent Laid-open No. Hei 11-87392). However, the method has room for improvement in that it involves complicated steps and it is disadvantageous from the viewpoints of speediness and production cost. Furthermore, the method has the problem that the non-conductive property of the remaining oxide film leads to an increased thermal resistance, resulting in worsening of electrical properties.
Removal of the rigid aluminum oxide film has been carried out by a wet method. Specifically, the oxide film has been removed by a method in which the workpiece is immersed in a strong alkaline solution or acidic solution so as to completely remove the oxide film while partly dissolving the underlying aluminum or aluminum alloy basis material. While this method may be satisfactory where the aluminum or aluminum alloy basis material is thick, it becomes very difficult to secure a margin of etching when the thickness of the aluminum or aluminum alloy basis material is reduced to 0.5 μM or 1.0 μm.
In addition, a method in which an organic solvent is used (refer to Japanese Patent Laid-open No. 2002-151537), a method in which a mixture of several acids is used (refer to Japanese Patent Laid-open No. Hei 5-65657 and JP-A-2002-514683) and the like have also been proposed.
However, in these methods, considerable etching of the aluminum or aluminum alloy basis material cannot be obviated, and, where the basis material is a thin film, the thin film would be lost or dissolved; therefore, it is difficult to select appropriate treatment conditions. Furthermore, a grinding or other mechanical polishing step cannot be adopted for the thin film, unlike the case of die casting. Therefore, the oxide film formed through a heat treatment in the manufacturing process would be left on the surface of the aluminum thin film, which worsens the situation.
Incidentally, Japanese Patent Laid-open No. 2004-263267 is also mentioned as a related-art document.